1. Field of the Invention
The invention relates generally to semiconductor thermal processing during fabrication. More particularly, the invention is directed to a system and method for filtering temperature anomalies from a closed-loop thermal control system.
2. Description of Related Art
Individual semiconductors or integrated circuit (IC) devices are typically formed on a semiconductor substrate by numerous different processes. A number of these processes require thermally processing the semiconductor substrate to allow various chemical and physical reactions to take place as the substrate is fabricated into multiple IC devices. The systems that perform the thermal processing typically include a heat source, a controller for controlling the heat source, and a substrate holder for holding the semiconductor substrate adjacent the heat source during thermal processing.
Modern thermal processing systems heat semiconductor substrates under controlled conditions according to predetermined thermal recipes. These thermal recipes fundamentally consist of a temperature that the semiconductor substrate must be heated to, and the time that the thermal processing system remains at that temperature. For example, thermal recipes may require the semiconductor substrate to be heated to distinct temperatures between 30 and 1200° C., for processing times at each distinct temperature of between 0 and 60 seconds.
To meet certain objectives, such as minimal diffusion, these thermal processing systems must also restrict the amount of time that each semiconductor substrate is subjected to a high temperature. To accomplish this, the temperature ramp rate is often very steep, i.e., the thermal processing system often needs to change from a high to low temperature, or visa versa, in as short a time as possible.
These steep temperature ramp rates led to the development of Rapid Thermal Processing (RTP). During RTP the semiconductor substrate is irradiated with a radiant heat source powerful enough to quickly raise the temperature of the semiconductor substrate to the desired process temperature and hold it at that temperature for a sufficient period of time to accomplish a specific process step. Typical temperature ramp rates for RTP range from 20-250° C./second as compared to 5-15° C./minute for conventional furnaces.
RTP is typically used for thermal oxidation, Chemical Vapor Deposition (CVD), substrate bonding, and annealing. What is more, RTP is rapidly becoming the technology of choice for oxidation and annealing steps in advanced ultra-large scale integration (ULSI) fabrication.
The radiant heat sources used in RTP systems are mostly tungsten-halogen or arc lamps arranged in a linear or circular array. This array is typically located directly above, below, or both above and below the semiconductor substrate. RTP systems typically also rotate the semiconductor substrate while irradiating it to more evenly distribute temperature across the surface of the semiconductor substrate.
Since these lamps have very low thermal mass relative to furnaces, the substrate can be heated rapidly. Rapid substrate cooling is also easily achieved since the heat source may be turned off quickly without requiring a slow temperature ramp-down. Lamp heating of the substrate minimizes the thermal mass effects of the process chamber and allows rapid real time control over the substrate temperature.
An example of a typical prior art RTP system 102 is shown in FIG. 1. An example of such a system is disclosed in Applicant's U.S. Pat. Nos. 5,660,472; 5,689,614; 5,755,511; 5,781,693; 6,123,766; 6,350,964; 6,395,363, all of which are incorporated herein by reference. Furthermore, Applicant's tools incorporating such a system are sold under the RADIANCE® brand.
The RTP system 102 is shown in an open or non-operational position. Such an RTP system 102 comprises a lid 104 housing a circular array of concentric zones of heating lamps 110, and a RTP chamber 108 housing a semiconductor substrate 106. These arrays typically have about two hundred lamps for a 200 mm semiconductor substrate.
Each concentric zone of heating lamps is separately controlled by a controller 114 that is used to heat the substrate according to a predetermined thermal recipe. Temperature probes (not shown) under the substrate are used to feedback a measured temperature to the controller 114. This allows the controller to adjust each zone of concentric heating lamps to ensure that the measured temperature reflects a desired temperature dictated by the thermal recipe.
Semiconductor substrates 106 typically include areas on the surface of the substrate, which are indicated by cross-hatching, where integrated circuits (ICs), such as transistors, have or will be formed (hereinafter “IC area/s”), and areas where no ICs are or will be formed (hereinafter “open area/s”) 112. The open area/s are formed for a number of reasons. For example, ICs typically cannot be formed at the substrate edge, or around other non-IC features on the substrate. Such non-IC features include: alignment or registration features on the substrate, which are used to align semiconductor masks; a laser scribe or other identification feature for identifying each substrate; a hole used to align the substrate in each processing tool; or the like. These features are described in further detail in relation to FIG. 2B.
In use, the semiconductor substrate is typically rotated at several to several hundred revolutions per minute. However, in a preferred embodiment, the semiconductor substrate is rotated at ninety revolutions per minute. Therefore, the temperature probes measure different areas of the substrate at each instant of time. Furthermore, as the IC area/s and open area/s may have significantly different thermal and optical properties, the open area/s may absorb more or less heat than the IC area/s. Therefore, the temperature probes adjacent the open area/s could measure significantly higher or lower temperatures. To account for this difference, i.e., different temperatures measured adjacent the open area/s, the controller 114 attempts to adjust the temperature of the heating lamps. This results in different segments of the IC area/s being exposed to different temperatures, which results in the IC area/s not being heated uniformly, as is typically required by the thermal recipe. Such non-uniformity leads to slip, warpage, and/or deformation of the substrate. Slip and/or warpage makes alignment of subsequent masks difficult, if not impossible, as each layer is built upon a previous layer. Alignment of each mask is made using alignment marks on the substrate, which if not always aligned with the mask, can ruin dies, significantly reduce yield, or even destroy entire substrates.
In addition, non-uniformity of the substrate leads to nonuniform material properties, such as alloy content, grain size, and dopant concentration. These nonuniform material properties degrade the circuitry and decrease IC device yield per semiconductor substrate.
One prior art system compensating for these substrate temperature non-uniformities caused by open area/s, uses low pass filters or notch filters for the temperature data for all temperature probes. For example, where the thermal recipe calls for a temperature of 1000° C., the temperature probes 1-6 and 8 read 1000° C., while temperature probe 7, at an open area, reads 1020° C. at 20% of all time steps and 1000° C. at 80% of all time steps. The low pass or notch filter will average such measured temperatures and yields a measured temperature of 1004° C. Therefore, the controller will lower the temperature of the lamps by 4° C. to 996° C. at the temperature probe 7 position. This is unacceptable, as the thermal profile is adjusted either above or below the temperature required by the thermal recipe.
In addition, prior art systems that utilize low pass filters slow down the temperature response of the controller and/or the radiation source, thereby reducing controllability of the thermal process. This hampers fast temperature ramp-up and ramp-down rates, which are required by most RTP thermal recipes.
Since the above described notch or low pass filters do not differentiate between critical temperature data at IC area/s and non-critical temperature data at open area/s, it is common practice to minimize the open area/s on the substrate by stepping out all masks to the edge of the substrate, thereby making the substrate as uniform as possible. This is extremely costly, and almost impossible for high-resolution masks.
In light of the above, there is a need for a system and method for filtering temperature anomalies from a closed-loop thermal control system, without sacrificing temperature response or compromising the thermal recipe.